Re: [PATCH v3 1/2] IOMMU/spinlock: Fix a bug found in AMD IOMMU initialization

From: "Xu, Quan" <quan.xu@intel.com>
To: Dario Faggioli <dario.faggioli@citrix.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	Jan Beulich <jbeulich@suse.com>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Subject: Re: [PATCH v3 1/2] IOMMU/spinlock: Fix a bug found in AMD IOMMU initialization
Date: Thu, 10 Mar 2016 06:12:51 +0000	[thread overview]
Message-ID: <945CA011AD5F084CBEA3E851C0AB28894B85FAB2@SHSMSX101.ccr.corp.intel.com> (raw)
In-Reply-To: <1457535578.3102.394.camel@citrix.com>

CC Kevin, 

On March 09, 2016 11:00pm, <dario.faggioli@citrix.com> wrote:
> On Wed, 2016-03-09 at 21:17 +0800, Quan Xu wrote:
> > pcidevs_lock should be held with interrupt enabled.
> >
> There's a message from Jan when he says:
> <<Well, I'd say something like "pcidevs_lock doesn't require interrupts to be
> disabled while being acquired".>>
> 
> :-O
> 
> > However there remains
> > an exception in AMD IOMMU code, where the lock is acquired with
> > interrupt disabled. This inconsistency might lead to deadlock.
> >
> I appreciate that 'might' is weaker than 'can'. Personally, I still dob't find this
> correct, or at least clear enough, referred to this patch, but I won't be in the
> way because of this.
> 
> > The fix is straightforward to use spin_lock instead. Also interrupt
> > has been enabled when this function is invoked, so we're sure
> > consistency around pcidevs_lock can be guaranteed after this fix.
> >
> And I also can't really get what "so we're sure consistency around pcidevs_lock
> can be guaranteed after this fix" actually means, but again, that's probably me,
> and it's fine.
> 
> However,
> 
> > Signed-off-by: Quan Xu <quan.xu@intel.com>
> > Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
> >
> This still stands **only** if the very first sentence "pcidevs_lock should be held
> with interrupt enabled" is changed to "pcidevs_lock doesn't require interrupts to
> be disabled while being acquired".
> 

Dario,
I am open for this change:).

When I read:
1. (look at "Lesson 3: spinlocks revisited.")
  https://www.kernel.org/doc/Documentation/locking/spinlocks.txt
2. comment inside check_lock(), in xen/common/spinlock.c, in Xen's codebase.
3. the "Linux Device Drivers, 3rd Edition" book , http://www.makelinux.net/ldd3/chp-5-sect-5 

I found that:
    - "We partition locks into IRQ-safe (__always__ held with IRQs disabled) and IRQ-unsafe (__always__ held with IRQs enabled) types".

It looks like Kevin's description is better. 

I also found that you mentioned in this thread:
"...  __except for very special situations__".
If it is true, I think your description is better.

Thanks for your time..:):)

Quan
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